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Applying SO4 in the Fall

Applications of sulfate-containing fertilizers can result in leached sulfate and limited crop availability if applied too early. However, there are several factors to consider when determining if sulfate leaching should be a concern with fall application. Because sulfate is a negatively charged ion (anion), and most soil particles are also negatively charged, sulfate can be repelled and leached with water through the soil profile.

Soil type

Soil type is the first concern when evaluating the leaching potential of sulfate. Coarse soil textures (sandy, loamy sand, sandy loam) have two inherent qualities that impact nutrient leaching. First, these soils do not have the same ability to retain nutrients (both cations and anions) that soils with higher clay and organic matter content do. This is reflected by low cation exchange capacity (CEC). Secondly, since these soils are coarsely textured, water and leachable nutrients move more readily through them. If you have a soil that fits into these parameters, you should avoid fall applications of SO4. However, if your soil texture is heavier, a fall application is well within sensible limits.

Climate

Leaching can only occur when water is actually moving through the soil profile. Soils in the Midwest normally freeze in the winter, limiting leaching losses. If you are in a region that does not experience winter freezing, leaching losses are likely to be more substantial if the ground is in fallow for an extended period.

Sulfur source

Sulfur sources have varying solubility and availability, which affects leaching potential. Following are leaching considerations for three common sulfur sources:

SO4:Since SO4 has ideal solubility, it has less sulfate leaching potential than other sulfur sources. Therefore, if other conditions allow, SO4 can be fall applied in scenarios described above without concerns over sulfate leaching.

Ammonium sulfate (AMS):Ammonium sulfate is one of the most soluble fertilizers on the market (300x more soluble than SO4) and therefore has significant leaching potential, not only for sulfate but for nitrate-nitrogen. Ammonium sulfate should not be fall applied.

Elemental sulfur: Elemental sulfur must undergo conversion to sulfate, which is dependent on multiple environmental factors, so while leaching isn’t a major concern, availability is, which typically takes up to a year from the time of application.

Bottom line and economics

There are obvious benefits to fall application; if SO4 can be blended with your other fall fertilizer applications, you can save money on a separate spring application. If you are considering a fall application of SO4 and are on a non-sandy soil, and have frozen/cold ground in the winter, you can fall apply.